A study of 511 mammal species finds that more social animals evolve bigger brains. Cat brain size has grown less than that of cows, rhinos, and dogs.

Atop the list of brain-size-evolvers -- by a long shot -- are humans and apes. Other animals that have evolved larger and larger brains over time are the wet-nosed primates (such as lemurs), toothed whales (such as dolphins and sperm whales), single-toed hooved animals (such as horses and donkeys), camels and llamas, and dogs.

Animal suborders that did not evolve significantly larger brains include cats, odd-toed hooved animals such as rhinos and tapirs, and grazing animals such as cows and deer.

Cats, in fact, are at the bottom of the brain-size-evolving list.

When plotted against the ability of each group of animals to form stable social groups, a correlation emerges. The more socially successful animals tend to be those that evolved larger brains over time.

"Groups of species whose brain size has been increasing most over evolutionary time are those that are typically very social," study co-author Susanne Shultz, PhD, tells WebMD in an email. Shultz is a fellow of the Institute of Cognitive and Evolutionary Anthropology at the University of Oxford, U.K.

But do their big brains really make these more socially skilled animals smarter, or are they just a bunch of big-headed louts that hang out together?

"Intelligence is one of the difficult questions," Shultz says. "There is evidence from living species that those with large brains are better at learning, developing novel behaviors, adapting to new environments, and so on."

So if brain-size evolution is linked to smarts, cats would rank below cows, which would rank below dogs.

It's doubtful, however, that Shultz's research will settle the are-dogs-smarter-than-cats question. Cat lovers can always argue that cats were created smart and simply didn't need to get any smarter. Or more social.

Shultz and co-author Robin Dunbar, PhD, report their findings in the early online edition of the Proceedings of the National Academy of Sciences.